Four different raw materials (olive-oil mill, pruning waste, rice straw and almond shell waste) used as additives were mixed with pepper plant wastes for composting purposes. Three bacterial strains isolated from prior composting processes and identified as Bacillus shackletonni, Streptomyces thermovulgaris and Ureibacillus thermosphaericus were added to every windrow and their effect on humic-like substances evolution was studied. In all the cases, an increase in the humification indices (humic to fulvic acids ratio, humification ratio and humification index) was obtained. On the other hand, inoculation significantly affected the humification indices, and differences due to both raw materials and microbial inoculants could be observed. When inoculated in almond shell and rice straw heaps respectively, B. shackletonni and U. thermosphaericus induced higher humification levels than those obtained for other raw material/inoculant combinations. Thus, the improvement of composting processes by means of inoculation seems to depend on properties of raw materials and microorganisms applied. (c) 2006 Elsevier Ltd. All rights reserved.